1. Technical Field
The present invention relates to a method and apparatus for data transmission in general, and in particular to a method and apparatus for transmitting data on a bus. Still more particularly, the present invention relates to a method and apparatus for adaptively compensating skews during data transmission using cables over extended distances.
2. Description of the Prior Art
The technique of using a group of wires typically known as a bus to transmit data has been in common use since the early days of electronic computers. There are many advantages in using a single interconnecting scheme such as a bus for a computer system. For example, various subsystems within a computer system can be interfaced with each other via a bus. In addition, new devices and peripherals can easily be added or even be ported between computer systems that use a common bus. Furthermore, because a single set of wires can be shared among multiple sources and destinations, the cost associated with a bus implementation is relatively low.
However, for computer systems where bus traffic or throughput demands on input and output are relatively high, it is a major challenge to design a bus system capable of meeting such demands in an efficient manner. With conventional design, data are generally driven on to a bus, using logic voltage levels to encode and transmit a data bit in every wire within the bus, and this method of data transmission is sufficient for relatively narrow buses. But when wider buses are needed to meet the ever increasing throughput requirements, the problem of capacitive and inductive coupling between adjacent parallel bus wires and simultaneous switching of a large number of drivers with consequent peak current demand, becomes more significant. Thus, transitional coding is used to facilitate the transmission process. The transitional coded method of data transmission typically has one or more groups of four signal paths, and single transitions are sent in sequence down one of the possible four signal paths. Each such transition thus conveys two bits of self-clocked information (or data). For extended distance transmission over cables, each such group of four signal paths can be implemented as four twisted pairs. Logic level signals are converted into low-voltage differential signalling levels (LVDS), sent over the corresponding twisted pair, and reconverted back to logic levels at the other end of the cable using LVDS driver/receiver circuits.
During transmission, skew due to faster or slower signal propagation along different twisted pair signal paths within a cable, or through driver/receiver circuits, may limit the rate at which transitions can be sent. This is because signals sent along different paths must always arrive in correct order. The present disclosure provides a method and apparatus for adaptively compensating for such skews during transmission using the transitional coding method of data transmission, particularly over long cables.
In accordance with a preferred embodiment of the present invention, a bus includes one or more groups of multiple signal paths. Each of the signal paths within a group is assigned a unique binary data value. During data transmissions, only one signal path within each group is activated at a time, and each activation of one of the signal paths represents its associated unique binary data value. Thus, a sequence of consecutive activations represents the same information as if the data were transmitted in parallel. Before signal transmission, a time delay between a transition on a first and a second of the signal lines is measured by utilizing a control frame. The time delay measurement is repeated to establish the relative time delays associated with all possible transitions on all the signal lines. In response to the measured time delays, an appropriate compensating delay is introduced accordingly before launching each transition on the signal lines. Such added delay compensates for the differences in delay (or skew) between the different transitions travelling along different signal paths.
All objects, features, and advantages of the present invention will become apparent in the following detailed written description.